Method for manufacturing precious metal alloys and precious metal alloys thus obtained

ABSTRACT

A method for manufacturing a part by alloying a precious metal with boron, wherein: a quantity of precious metal reduced to powder form is provided; a quantity of a nano-structured micrometric boron powder is provided; the precious metal powder is mixed with the nano-structured micrometric boron powder to obtain a mixture; the mixture is compacted by applying a uniaxial pressure; the mixture is subjected to a spark plasma sintering or flash sintering treatment, or to a hot isostatic pressing (HIP) treatment, to obtain an ingot of a precious metal/boron alloy, and the ingot is machined to obtain the part, or the ingot is reduced to powder form by a micronisation treatment and the powder is treated to obtain the part. Additionally, a gold/boron alloy.

This application is a continuation of U.S. application Ser. No. 17/058,424 filed Nov. 24, 2020, pending, which is a National Stage of PCT/EP2019/083802 filed Dec. 5, 2019 and claims the benefit of EP 18210947.0 filed Dec. 7, 2018.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for manufacturing precious metal alloys. The present invention further relates to such precious alloys. In particular, the present invention relates to a method for manufacturing light precious metal alloys obtained from gold, silver, platinum, palladium, ruthenium, rhodium or iridium. The light precious metal alloys concerned hereby are titratable, i.e. they are alloys whose ratio between the weight of precious metal in the composition of the alloy and the total weight of this alloy is determined by law.

BACKGROUND OF THE INVENTION

A metal alloy results from the combination, by melting, of a first metallic element with at least one second metallic element. The advantage of metal alloys lies in the fact that the properties, in particular the mechanical properties, of such alloys are enhanced compared to the mechanical properties of the metallic elements constituting same taken individually.

The mechanical properties of a metal can in particular be enhanced by deformation, in particular by work hardening; these mechanical properties can also be enhanced by a chemical approach, by adding one or more alloying elements to the base metal. These additions often also enhance the chemical properties such as the resistance of the base metal to corrosion.

The metal alloy technique is of particular interest in the case of precious metals such as gold. More specifically, gold is known to be easily deformed when cold, which is why it has been used since the end of the Neolithic Period to produce jewellery and finery as well as coins from Antiquity. Nonetheless, the ease with which gold can be deformed is also a drawback since a simple mechanical impact is all it takes to deform the jewellery produced using this noble metal. This is why efforts were made very early on to enhance the mechanical properties of gold by alloying it with other metallic elements; silver and copper are the two primary metals used for alloying gold and are known to improve the stiffness of gold.

Alloying gold with other metallic elements such as silver or copper produces metal alloys with a hardness that is greater than that of gold. Nonetheless, these gold alloys have the drawback of having a high density. This is why attempts have been made to alloy gold with metallic elements having a lower density.

Tests have already been carried out to attempt to alloy gold (Au) which is a heavy metal, i.e. a metal with a high density (about 19.3 g·cm⁻³), with boron (B) which is a very light metal, i.e. with a low density (about 2.3 g·cm⁻³). Nonetheless, attempts made to date to try to alloy gold and boron using conventional metallurgy techniques have all resulted in failure or, at best, resulted in very low boron dissolution rates, which do not allow for industrial production. The materials resulting from the combination of gold and boron were seen to be unstable and the production of solid titratable components such as 18-karat gold using this combination was shown to be impossible. These problems are in particular due to the fact that, when melting, the gold and boron cannot be mixed; more specifically, as a result of the high density thereof, gold tends to sediment at the bottom of the crucible, whereas boron, whose density is lower, floats.

The recent marketing of boron powders obtained by nanostructuring techniques has revived the interest in gold and boron alloys and, more generally, in all types of alloy formed between a precious metal (gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh) or iridium (Ir)) and boron (B).

Methods for manufacturing metal alloys using powder metallurgy technology procure materials that would be impossible to manufacture using conventional metallurgy techniques. This is of particular interest in cases where the metal used to form such a metal alloy is a titratable precious metal such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh) or iridium (Ir). Moreover, the metal alloys obtained using powder technology are both lighter and harder than the metal alloys obtained using conventional metallurgy approaches.

A type of nano-structured boron takes the form of a grey/black-coloured powder formed of particles whose dimensions lie in the range 5 nm to 12 nm and which are formed of a core constituted of HfB₂, of NiB, of CoB, of YB₄ or of YB₆ and whose structure is crystalline, and of a layer of amorphous boron, the thickness thereof being equal to several nanometres and which coats the core of these particles. These particles are agglomerated with one another to form three-dimensional structures, the dimensions thereof lying in the micrometric range and the specific surface area thereof being in the order of 700 m² for 1 g of powder.

SUMMARY OF THE INVENTION

A purpose of the present invention is to procure a method for manufacturing light precious metal alloys in particular for obtaining light alloys that are stable from a physical-chemical standpoint with which solid components can be produced. The present invention further relates to such light precious alloys.

For this purpose, the present invention relates to a method for manufacturing a part by alloying a precious metal with boron, the precious metal being selected from the group formed by gold, silver, platinum, palladium, ruthenium, rhodium and iridium, this method comprising the steps of:

-   -   providing at least one quantity of precious metal reduced to         powder form;     -   providing a quantity of a nano-structured micrometric powder         formed by aggregates of particles, these aggregates which         constitute the nano-structured micrometric powder having         dimensions within the micrometric range, these aggregates being         themselves constituted of particles containing, at least in         part, boron, and which have at least one geometrical value         within the nanometric range responsible for the reactivity of         the nano-structured micrometric powder;     -   mixing the precious metal powder with the nano-structured         micrometric powder and compacting this mixture of powders by         applying a uniaxial pressure;     -   subjecting the mixture of precious metal powder and         nano-structured micrometric powder to a spark plasma sintering         or flash sintering treatment at a pressure that lies in the         range 0.5 GPa to 10 GPa, or to a hot isostatic pressing (HIP)         treatment at a pressure that lies in the range 80 bar to 2,200         bar, the treatment being carried out at a temperature that lies         in the range 400° C. to 2,100° C. in order to obtain at least         one precious metal/boron alloy ingot, and     -   machining the precious metal/boron alloy ingot to obtain the         desired part, or     -   reducing the precious metal/boron alloy ingot to powder form by         a micronisation treatment, and obtaining the desired part by         treating the powder resulting from the micronisation treatment.

Thanks to these features, the method according to the invention procures precious metal/boron alloys which have both excellent mechanical properties and whose density is low. To the Applicant's knowledge, the method according to the invention offers, for the first time, the possibility of alloying, on an industrial scale, a component of a very low density, in this instance boron, with a precious metal, in particular gold, however not limited thereto, the density thereof being high. Thanks to the method of the invention, precious metal/boron alloys can be obtained which are stable from a physical-chemical standpoint, having excellent mechanical properties, and whose density is low. In a remarkable manner, in the method according to the invention, the precious metal selected and the boron particles are closely alloyed, without any segregation phenomenon between the two materials being observed at any time.

According to special embodiments of the invention:

-   -   in order to obtain the desired part, the powder resulting from         the micronisation treatment is inserted into a mould and         subjected to a uniaxial or isostatic pressure;     -   in order to obtain the desired part, the powder resulting from         the micronisation treatment is subjected to a three-dimensional         additive manufacturing treatment;     -   the three-dimensional additive manufacturing treatment is of the         direct printing type;     -   the treatment by direct printing is selected from the group         formed by selective laser melting (SLM) and E-beam melting;     -   the three-dimensional additive manufacturing treatment is of the         indirect printing type;     -   the indirect printing treatment is selected from the group         formed by Inkjetting, nanoparticle jetting (NPJ) and Digital         Light Projecting (DLP).

According to another special embodiment of the invention, the manufacturing method further comprises the steps of:

-   -   mixing the powder resulting from the micronisation treatment of         the precious metal/boron alloy ingot with a binder to obtain a         feedstock;     -   producing a green body by subjecting the feedstock to an         additive manufacturing injection or micro-injection;     -   obtaining a brown body by subjecting the green body to a step of         eliminating the polymer binder, referred to as a debinding step,         during which the green body is chemically treated then heat         treated in a furnace to burn the residual polymer binder, this         debinding step typically being carried out in gaseous phase in a         nitric acid or oxalic acid atmosphere and at a temperature in         the range 100° C. to 140° C.;     -   subjecting the brown body to a sintering treatment in a         protected atmosphere and at a temperature in the range 700° C.         to 1,800° C. to obtain the desired part.

According to another special embodiment of the invention, the manufacturing method further comprises the steps of:

-   -   mixing the powder resulting from the micronisation treatment of         the precious metal/boron alloy ingot with a binder to obtain a         feedstock;     -   producing a green body, the shape thereof corresponding to the         desired work profile using an indirect additive manufacturing         technique;     -   obtaining a brown body by subjecting the green body to a step of         eliminating the polymer binder, referred to as a debinding step,         during which the green body is chemically treated then heat         treated in a furnace to burn the residual polymer binder, this         debinding step typically being carried out in gaseous phase in a         nitric acid or oxalic acid atmosphere and at a temperature in         the range 100° C. to 140° C.;     -   subjecting the brown body to a sintering treatment in a         protected atmosphere and at a temperature in the range 700° C.         to 1,800° C. to obtain the desired part.

According to other special embodiments of the invention:

-   -   the additive manufacturing technique is selected from the group         formed by Binder jetting, Solvent on Granulate jetting, FDM or         micro-extrusion;     -   after the sintering treatment, the part resulting from the         sintering step is subjected to a hot isostatic pressing (HIP)         post-treatment step at a pressure that lies in the range 500 bar         to 2,200 bar, and at a temperature that lies in the range         600° C. to 2,100° C.;     -   the binder is selected from the group formed by polyethylene         glycol (PEG), cellulose acetate butyrate (CAB), nano-cellulose,         corn starch, sugar, polylactic acid (PLA), polyethylene,         polypropylene, synthetic or natural wax and stearic acid;     -   the precious metal is selected from the group formed by gold         (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium         (Ru), rhodium (Rh) and iridium (Ir), and the nano-structured         micrometric boron powder is selected from the group formed by         NiB, CoB, YB₄ and YB₆;     -   25 wt % nano-structured micrometric boron powder is mixed with         75 wt % gold;     -   a first form of nano-structured micrometric boron powder well         suited to the needs of the present invention takes the form of a         black/brown-coloured powder, formed by particles whose         dimensions lie in the range 25 to 999 nm and which are made of         boron in proportions by weight of greater than 95%, the         remainder being made of unavoidable impurities incorporated         during synthesis, such as, in a non-limiting manner, carbon,         nitrogen, oxygen, potassium, sodium, chlorine, iodine, caesium,         lithium, rubidium and magnesium. One example of a         nano-structured micrometric boron powder is given by the product         marketed by Pavezyum under the reference PVZ nano Boron;     -   a second form of nano-structured micrometric boron powder well         suited to the needs of the present invention corresponds to the         material referred to by the name “nano-structured amorphous         boron material”, the features and conditions for obtaining same         are described in the international patent application PCT         WO2016207558A1. This is a nano-structured micrometric boron         powder taking the form of particles whose dimensions do not         exceed 25 nanometres and which are made of boron in proportions         by weight of greater than or equal to 85%, the remainder being         constituted by unavoidable impurities originating from the         synthesis of the powder which are one or more of the following         elements: lithium, sodium, potassium, rubidium, caesium, iodine,         chlorine, bromine and fluorine. This nano-structured micrometric         boron powder is obtained by synthesis by molten salt (SMS), this         synthesis being carried out by a dry process, by a wet process         or in an argon atmosphere;     -   a third form of nano-structured micrometric boron powder well         suited to the needs of the present invention takes the form of a         grey/black-coloured powder formed of particles whose dimensions         lie in the range 5 nm to 12 nm and which are formed of a core         constituted of HfB₂, of NiB, of CoB, of YB₄ or of YB₆ and whose         structure is crystalline, and of a layer of amorphous boron, the         thickness thereof being equal to several nanometres and which         coats the core of these particles. These particles are         agglomerated with one another to form aggregates, the dimensions         thereof lying in the micrometric range and the specific surface         area thereof being in the order of 700 m² for 1 g of powder.         This nano-structured micrometric boron powder is obtained by         synthesis by molten salt (SMS), this synthesis being carried out         by a dry process, by a wet process or in an argon atmosphere;     -   the gold/boron alloy according to the method of the invention         allows an 18-karat gold to be obtained that has a density in the         range 6.6 to 7 g/cm³.

The particle size is determined by one of the following techniques: transmission electron microscopy, scanning electron microscopy, dynamic light scattering or X-ray diffraction (using the Scherrer equation as regards crystalline materials).

The specific surface area of the powders is determined using the nitrogen adsorption technique, according to the Brunauer-Emmet-Teller (BET) method.

Finally, the density is determined using a pycnometer.

DETAILED DESCRIPTION OF ONE METHOD OF IMPLEMENTING THE INVENTION

The present invention was drawn from the general inventive idea consisting of procuring a titratable precious metal alloy that is stable from a physical-chemical standpoint and which has excellent mechanical properties.

For this purpose, the present invention relates to a method for manufacturing a part by alloying a precious metal with boron, the precious metal being chosen from the group formed by gold, silver, platinum, palladium, ruthenium, rhodium and iridium, this method comprising the steps of:

-   -   providing at least one quantity of precious metal reduced to         powder form;     -   providing a quantity of a nano-structured micrometric powder         formed by aggregates of particles, these aggregates which         constitute the nano-structured micrometric powder having         dimensions within the micrometric range, these aggregates being         themselves constituted of particles containing, at least in         part, boron, and which have at least one geometrical value         within the nanometric range responsible for the reactivity of         the nano-structured micrometric powder;     -   mixing the precious metal powder with the nano-structured         micrometric powder and compacting this mixture of powders by         applying a uniaxial pressure;     -   subjecting the mixture of precious metal powder and         nano-structured micrometric powder to a spark plasma sintering         or flash sintering treatment at a pressure that lies in the         range 0.5 GPa to 10 GPa, or to a hot isostatic pressing (HIP)         treatment at a pressure that lies in the range 80 bar to 2,200         bar, the treatment being carried out at a temperature that lies         in the range 400° C. to 2,100° C. in order to obtain at least         one precious metal/boron alloy ingot, and     -   machining the precious metal/boron alloy ingot to obtain the         desired part, or     -   reducing the precious metal/boron alloy ingot to powder form by         a micronisation treatment, and obtaining the desired part by         treating the powder resulting from the micronisation treatment.

Once the precious metal/boron alloy ingot obtained by implementing the method according to the invention has been micronised, a first possibility for obtaining the desired solid part consists of inserting the powder resulting from the micronisation treatment into a mould and of subjecting this mould to a uniaxial or isostatic pressure.

Once the precious metal/boron alloy ingot obtained by implementing the method according to the invention has been micronised, a second possibility for obtaining the desired solid part consists of subjecting the powder resulting from the micronisation treatment to a three-dimensional additive manufacturing treatment.

The three-dimensional additive manufacturing treatment can be of the direct printing type. The available three-dimensional additive manufacturing techniques of the direct type are selective laser melting (SLM) and E-beam melting.

The three-dimensional additive manufacturing treatment can be of the indirect printing type. The available three-dimensional additive manufacturing techniques of the indirect type are:

-   -   inkjetting: the powder resulting from the micronisation         treatment of the precious metal/boron alloy ingot is dispersed         in the ink. The ink is printed layer after layer, each layer         being hardened by exposure to the rays of a light source for         example UV light before depositing the subsequent layer.     -   nanoparticle jetting (NPJ): this technique, in particular         developed by XJet, is similar to liquid inkjetting, with the         exception that the ink is made of suspended nanoparticles         resulting from the micronisation treatment. The suspension is         then sprayed and dried layer by layer.     -   Digital Light Projecting (DLP): this technique consists of         projecting by reflection on a mirror, the image of the part to         be structured on a powder bed containing the powder particles         resulting from the micronisation treatment dispersed in a         photopolymer.

According to another special embodiment of the invention, after having mixed the powder resulting from the micronisation treatment of the precious metal/boron alloy ingot with a binder to obtain a feedstock, a green body is produced, the shape thereof corresponding to the desired work profile by subjecting the feedstock either to an injection or micro-injection, or to an additive manufacturing technique.

The available indirect additive manufacturing techniques include:

-   -   Binder jetting: this technique consists of spraying an inkjet         containing a solvent and a binder on a powder bed in which the         powder particles resulting from the micronisation treatment are         dispersed.     -   Solvent on Granulate jetting: this technique consists of         spraying a solvent on a granulate bed, each of these granulates         being formed by a plurality of powder particles resulting from         the micronisation treatment agglomerated with one another by         means of a binder. The dimensions of these granulates are in the         order of 10 μm to 50 μm. The desired part is printed, layer by         layer, with the granulates agglomerating thanks to the binder.         Once the desired part has been printed, it is subjected to a         debinding operation in order to eliminate the solvent, and is         then sintered.     -   FFD (Fused Filament Deposition): filaments whose dimensions lie         in the millimetric range are produced by agglomerating the         powder particles resulting from the micronisation treatment by         means of a binder. These filaments are then heated and the         material of which they are made escapes from a nozzle, the         diameter whereof is in the order of 40 μm and allow the desired         part to be printed in three dimensions.     -   micro-extrusion.

The binder is selected from the group formed by polyethylene glycol (PEG), cellulose acetate butyrate (CAB), nano-cellulose (nanometric derivative of cellulose), corn starch, sugar, polylactic acid (PLA), polyethylene, polypropylene, synthetic or natural wax and stearic acid.

A brown body is obtained by subjecting the green body to a step of eliminating the polymer binder referred to as a debinding step during which the green body is chemically treated then heat treated in a furnace to burn the residual polymer binder, this debinding step typically being carried out in gaseous phase in a nitric acid or oxalic acid atmosphere and at a temperature in the range 100° C. to 140° C.

Finally, the brown body is subjected to a sintering treatment in a protected atmosphere and at a temperature in the range 700° C. to 1,800° C. to obtain the desired part.

It should be noted that, after the sintering treatment, the part resulting from the sintering step can be subjected to a hot isostatic pressing (HIP) post-treatment step at a pressure that lies in the range 500 bar to 2,200 bar, and at a temperature that lies in the range 600° C. to 2,100° C.

According to specific embodiments of the invention, the precious metal is chosen from the group consisting of gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh) and iridium (Ir). The boron is selected from the group formed by:

-   -   boron which takes the form of a black/brown-coloured powder,         formed by particles whose dimensions lie in the range 25 to 999         nm and which are made of boron in proportions by weight of         greater than 95%, the remainder being made of unavoidable         impurities incorporated during synthesis, such as, in a         non-limiting manner, carbon, nitrogen, oxygen, potassium,         sodium, chlorine, iodine, caesium, lithium, rubidium and         magnesium. One example of such a material is given by the         product marketed by Pavezyum under the reference PVZ nano Boron;     -   boron whose features and conditions for obtaining same are         described in the international patent application PCT         WO2016207558A1. This boron takes the form of a powder formed by         particles whose dimensions do not exceed 25 nanometres and which         are made of boron in proportions by weight of greater than or         equal to 85%, the remainder being constituted by unavoidable         impurities originating from the synthesis of the powder which         are one or more of the following elements: lithium, sodium,         potassium, rubidium, caesium, iodine, chlorine, bromine and         fluorine.     -   grey/black-coloured nano-structured micrometric boron powder         formed of particles whose dimensions lie in the range 5 nm to 12         nm and which are formed of a core constituted of HfB₂, of NiB,         of CoB, of YB₄ or of YB₆ and whose structure is crystalline, and         of a layer of amorphous boron, the thickness thereof being equal         to several nanometres and which coats the core of these         particles. These particles are agglomerated with one another to         form aggregate structures, the dimensions thereof lying in the         micrometric range and the specific surface area thereof being in         the order of 700 m² for 1 g of powder. To obtain the desired         material, one ratio that procures good results for the gold and         boron mixture is 25 wt % nano-structured micrometric boron         powder and 75 wt % gold.

In particular, the present invention relates to a method for manufacturing light precious metal alloys obtained from gold, silver, platinum, palladium, ruthenium, rhodium or iridium. The light precious metal alloys concerned hereby are titratable, i.e. they are alloys whose ratio between the weight of precious metal in the composition of the alloy and the total weight of this alloy is determined by law. A remarkable precious metal alloy obtained thanks to the method of the invention is an 18-karat gold/boron alloy with a density in the range 6.6 to 7 g/cm³. The products obtained are AuB₆ and Au₂B₁₂, the latter component being capable of containing AuB₂ inclusions.

It goes without saying that the present invention is not limited to the embodiment described above and that various simple alternatives and modifications can be considered by a person skilled in the art without leaving the scope of the invention as defined by the accompanying claims.

It should in particular be noted that it has been decided to qualify the boron powders concerned hereby as “micrometric” and “nano-structured” insofar as the particle aggregates constituting these powders have dimensions within the micrometric range, these aggregates being themselves constituted of particles essentially containing boron and which have at least one geometric value within the nanometric range responsible for the reactivity of the nano-structured micrometric powder. The term geometric value responsible for reactivity is understood to mean a geometric value such as the specific surface area, the surface roughness, or even the area of one face of the boron-containing particles which is located in the nanometric range. Thus, according to the invention, although the boron-containing particles have at least one geometrical value within the nanometric range, other geometrical dimensions of these particles can lie in the micrometric range, without this affecting the reactivity of these boron-containing particles. It should also be noted that, throughout the present description, the term nano-structured micrometric powder is understood to mean a material formed by aggregates which extend in the micrometric range and which result from the agglomeration of particles essentially containing boron, and at least one dimension thereof lying in the nanometric range.

It should also be noted that, although the present description essentially concerns a binary precious metal alloy formed of gold and boron, the present invention is not limited to such an example and also includes precious metal alloys that are, for example, ternary or quaternary. For the purposes of illustration, according to the invention, particles of nano-structured micrometric boron powder can be mixed with gold in the respective weight percentages of 75 wt % and 23 wt %, with the remainder being constituted by micronised nickel.

It should also be noted that the gold used within the scope of this invention is 24-karat ½ bright yellow gold and that the dimensions of the particles obtained by hammering this gold and which form the gold powder used within the scope of the present invention are less than 50 μm.

It should also be noted that the nano-structured micrometric boron particles concerned hereby are in particular known from the thesis by Remi Grosjean, entitled “Boron-based nanomaterials under extreme conditions”, pages 70 et seq., presented publicly on 17 Oct. 2018 at Université Pierre et Marie Curie—Paris VI, 2016. This thesis can be accessed over the internet at the following address: https://tel.archives-ouvertes.fr/tel-01898865 (HAL Id: tel-01898865). These nano-structured micrometric boron particles are obtained by synthesis in molten salts (SMS). This synthesis consists of placing metal and boron in the presence of the reactive substances in a salt mixture. When the mixture is heated, the salts melt, thus acting as a liquid medium. The typical synthesis of nano-structured borides in molten salts involves a metal source (usually a chloride MC10, and sodium borohydride. Sodium borohydride is used both as a boron source and as a reducing agent in order to obtain M° in the reaction medium. The use of such precursors and of lithium and potassium salts requires working under an inert atmosphere due to the sensitivity of these chemicals to water and/or oxygen. As a result, the precursors are handled and mixed in a laboratory glove box under an argon inert atmosphere. The synthesis itself is carried out under an argon atmosphere and not under a nitrogen atmosphere, since the nitrogen is capable of reacting with certain boron species and of yielding boron nitride.

The requirements for the experimental set-up are thus the following:

-   -   ensuring that the reaction medium is kept under an argon         atmosphere; this objective is achieved by using a quartz tube         which is stable at the working temperatures and which is         connected to a Schlenk line.     -   heating in a temperature range of between 300° C. and 1,000° C.         The area of the quartz tube in which the temperature is         homogeneous is about 8 cm wide, i.e. it is large enough to allow         homogeneous heating of the reaction medium, and low enough to         allow the salt vapours to condense to prevent solvent loss         during the reaction. The heating rate is 10° C./minute.     -   preventing side reactions between the reaction medium and the         quartz tube. Vitreous carbon is chemically inert under an argon         atmosphere and is thus used as a crucible. The crucible is long         enough to allow the salt vapours to condense in the         low-temperature area of the furnace.     -   in some cases, the reaction mixture must be stirred. This takes         place using a rotating vitreous carbon rod.

After the reaction, the reaction medium is left to cool naturally. Metal borides are obtained in the form of nanoparticles dispersed in a volume of frozen salts. To remove the salts, washing/centrifugation cycles are carried out in a polar solvent such as water or methanol. The adjustable parameters notably include the synthesis temperature, the dwell time and the initial ratio between the metal and boron sources.

The aforementioned thesis particularly concerns two nano-structured metal borides: hafnium diboride and calcium hexaboride. CaB6 and HfB2 have no phase transition under a high temperature and high pressure and are thus well suited for studying crystallisation of the amorphous phase in which the boride particles are embedded.

Two mixtures of eutectic salts, i.e. LiCl/KCl and Lil/KI, were used. The first syntheses of HfB2 was conducted in a LiCl/KCl (45/55 wt %) eutectic mixture, for which the melting point is about 350° C. HfCl4 and NaBH4 are used in a Hf:B=1:4 molar ratio and are mixed with the salt solution. After heating at 900° C. for 4 hours, cooling, washing with deionised water and drying under a vacuum, a black powder is obtained. The X-ray diffraction pattern of this powder shows that HfB2 is the only crystalline phase and exhibits no reflections corresponding either to the solvent salts or to the sodium chloride which can occur as side products of the boride formation. Moreover, the HfB2 structure is typical of that of diborides with metal atoms intercalated between boron sheets exhibiting a honeycomb structure.

According to the Scherrer equation, the particle size is 7.5 nm. This is confirmed by transmission electron microscopy which shows that the particle size lies in the range 5-12 nm. Other images obtained by SAED, FFT and HRTEM confirm that HfB2 is the sole crystalline phase in the material and that only the nanoparticles are crystalline.

Transmission electron microscopy also shows that the particles are surrounded by an amorphous shell, the thickness thereof lying in the range 2 to 4 nm. The particles are present in the form of inclusions inside a three-dimensional amorphous matrix. The gaps between the particles are filled with an amorphous matrix, the thickness thereof lying in the range 2 to 4 nm. As a result, the matrix is also nano-structured and the material can be described as a nanocomposite. 

1. A method for manufacturing a part by alloying a precious metal with boron, the precious metal being selected from the group consisting of gold, silver, platinum, palladium, ruthenium, rhodium and iridium, the method comprising: providing a quantity of the precious metal reduced to powder form; providing a quantity of a powder formed by aggregates of micrometric sized particles, and wherein the aggregates are composed of nanometric sized particles comprising boron that is responsible for a reactivity of the powder; mixing the precious metal reduced to powder form with the powder, to obtain a mixture; compacting the mixture by applying a uniaxial pressure; subjecting the mixture to a spark plasma sintering or flash sintering treatment at a pressure in a range of from 0.5 GPa to 10 GPa and at a temperature in a range of from 400° C. to 2,100° C., or to a hot isostatic pressing (HIP) treatment at a pressure in a range of from 80 bar to 2,200 bar and at a temperature in a range of from 400° C. to 2,100° C., to obtain an ingot of an alloy comprising precious metal and boron, and machining the ingot to obtain the part, or reducing the ingot to a powder by a micronisation treatment and treating the powder to obtain the part.
 2. The method of claim 1, wherein, to obtain the part, the ingot is reduced to the powder by the micronisation treatment and the powder is inserted into a mould and subjected to a uniaxial or isostatic pressure.
 3. The method of claim 1, wherein, to obtain the part, the ingot is reduced to the powder by the micronisation treatment and the powder is subjected to a three-dimensional additive manufacturing treatment.
 4. The method of claim 3, wherein the three-dimensional additive manufacturing treatment is a direct printing treatment.
 5. The method of claim 4, wherein the direct printing treatment is selected from the group consisting of selective laser melting (SLM) and electron beam (E-beam) melting.
 6. The method of claim 3, wherein the three-dimensional additive manufacturing treatment is an indirect printing treatment.
 7. The method of claim 6, wherein the indirect printing treatment is selected from the group consisting of inkjetting, nanoparticle jetting (NPJ) and Digital Light Projecting (DLP).
 8. The method of claim 1, wherein the ingot is reduced to the powder by the micronisation treatment and the method further comprises: mixing the powder with a binder, to obtain a feedstock; producing a green body, a shape thereof corresponding to a part profile, by subjecting the feedstock either to an injection or micro-injection, or to an additive manufacturing technique; obtaining a brown body by subjecting the green body to eliminating of the binder, referred to as debinding, during which the green body is chemically treated then heat treated in a furnace to burn residual binder, the debinding being carried out in gaseous phase in a nitric acid or oxalic acid atmosphere and at a temperature in a range of from 100° C. to 140° C.; and subjecting the brown body to a sintering treatment in a protected atmosphere and at a temperature in a range of from 700° C. to 1,800° C. to obtain the part.
 9. The method of claim 8, wherein the green body is produced by subjecting the feedstock to the additive manufacturing technique and the additive manufacturing technique is selected from the group consisting of binder jetting, solvent on granulate jetting, FDM or micro-extrusion.
 10. The method of claim 8, wherein after the sintering treatment, the part is subjected to a hot isostatic pressing (HIP) post-treatment at a pressure in a range of from 500 bar to 2,200 bar, and at a temperature in a range of from 600° C. to 2,100° C.
 11. The method of claim 8, wherein the binder is selected from the group consisting of polyethylene glycol (PEG), cellulose acetate butyrate (CAB), nano-cellulose, corn starch, sugar, polylactic acid, polyethylene, polypropylene, synthetic wax, natural wax and stearic acid.
 12. The method of claim 11, wherein the precious metal is gold.
 13. The method of claim 1, wherein the gold is 24-karat yellow gold and is in the form of particles having dimensions of less than 50 μm.
 14. The method of claim 1, wherein the nano-structured micrometric boron powder is selected from the group consisting of NiB, CoB, YB₄ and YB₆.
 15. The method of claim 14, wherein the nano-structured micrometric boron powder is formed of particles whose dimensions are in a range of from 5 nm to 12 nm and which are formed of a core constituted by HfB₂, whose structure is crystalline, and of a layer of amorphous boron, wherein the layer coats the core and has a thickness of several nanometres, and the particles are agglomerated with one another to form three-dimensional structures, the dimensions of which are in a micrometric range.
 16. The method of claim 15, wherein a specific surface area of the nano-structured micrometric boron powder is 700 m² for 1 g of powder.
 17. The method of claim 1, wherein the nano-structured micrometric powder is formed of particles whose geometrical dimensions are in a range of from 25 to 999 nm and which are made of boron in proportions by weight of greater than 95%, the remainder being constituted by unavoidable impurities originating from synthesis of the nano-structured micrometric powder and which are one or more of the following elements: carbon, nitrogen, oxygen, potassium, sodium, chlorine, iodine, caesium, lithium, rubidium and magnesium, the particles having a geometric value in a nanometric range responsible for a reactivity of the nano-structured micrometric powder.
 18. The method of claim 17, wherein a specific surface area of the nano-structured micrometric boron powder is at least 100 m² for 1 g of powder.
 19. The method of claim 1, wherein the nano-structured micrometric powder takes the form of particles whose dimensions do not exceed 25 nm and which are made of boron in proportions by weight of greater than or equal to 85%, the remainder being constituted by unavoidable impurities originating from synthesis of the nano-structured micrometric powder and which are one or more of the following elements: lithium, sodium, potassium, rubidium, caesium, iodine, chlorine, bromine and fluorine.
 20. An AuB₆ or Au₂B₁₂ gold/boron alloy, the latter component being capable of comprising AuB₂ inclusions.
 21. The alloy of claim 20, which is 18-karat gold.
 22. The alloy of claim 20, which comprises other alloyed elements.
 23. The alloy of claim 20, which has a density in a range of from 6.6 to 7 g/cm³.
 24. The method of claim 9, wherein after the sintering treatment, the part is subjected to a hot isostatic pressing (HIP) post-treatment at a pressure in a range of from 500 bar to 2,200 bar, and at a temperature in a range of from 600° C. to 2,100° C.
 25. The method of claim 13, wherein the nano-structured micrometric boron powder is selected from the group consisting of NiB, CoB, YB₄ and YB₆.
 26. The method of claim 25, wherein the nano-structured micrometric boron powder is formed of particles whose dimensions are in a range of from 5 nm to 12 nm and which are formed of a core constituted by HfB₂, whose structure is crystalline, and of a layer of amorphous boron, wherein the layer coats the core and has a thickness of several nanometres, and the particles are agglomerated with one another to form three-dimensional structures, the dimensions of which are in a micrometric range.
 27. (The alloy of claim 21, which has a density in a range of from 6.6 to 7 g/cm³.
 28. The alloy of claim 22, which has a density in a range of from 6.6 to 7 g/cm³. 